Humanoid bipedal robots have been known in general as basically human-shaped robots. Like human beings, they have two legs extending from a hip at the lower end of an upper body (torso), and may have two arms extending from shoulders on the left and right opposite sides of the upper body. These humanoid robots maintain balance by having large feet and actively controlling their body posture. By adjusting the posture, they position their center directly above the foot touching the ground to achieve stability. This is made easier with large feet that provide large contact areas with the ground. In most embodiments, such robots rely on inertial sensing to actively control the location of the center of gravity and maintain it above the feet.
Such balancing control strategies provide static stability, i.e. the robot maintains its balance throughout its walking gait. Its movement can be interrupted at any time without loss of stability. One example is the zero moment point (ZMP) strategy, commonly used in humanoids. Alternatively, other approaches to bipedal locomotion use dynamic stability, in which the robot may transition through a series of stable states, passing through periods of instability during transitions between states. Conventional biped robots have multiple electric or hydraulic motors installed in their legs, such as at the hip, knee and ankle. Consecutive segments of the leg are typically connected by motor-gear articulations that enable motions of these segments one relative to the other. The coordinated actuation of these articulations makes the leg describe specific trajectories and generate locomotion gaits. High level locomotive control may be based on either motion or torque/force. If based on motion, actuation of the motor-gear articulations creates specific motion trajectories that generate locomotion gaits. If based on torque/force, joint-level motor-gear articulations are driven with particular output torque or force trajectories, which interact with body dynamics and the environment (e.g. the ground) to produce the desired stable progression of the robot's center of mass.
The limitations of conventional biped design are twofold. A first limitation of conventional biped design is that conventional drive system designs used for walking robots are not power efficient. Significant energy is lost due to friction, excessive acceleration of drivetrain elements, and inefficient conversion between physical domains (e.g. electrical to mechanical or electrical to hydraulic to mechanical) in the actuators. In conventional designs, friction consumes energy in the actuators (e.g. hydraulic, pneumatic, or electric), transmissions, or joints. Large gear ratios (e.g. 50:1 or greater) are commonly used, and this requires very large accelerations of the actuators and elements of the transmissions that are upstream of the gear reduction whenever the joint speed changes (which happens almost constantly during walking); these internal accelerations require large actuator torques. Much energy is typically also lost in the conversion of source energy (e.g. electrical energy from a battery) to mechanical work done on the joints. For example, typical servo-hydraulic actuators are extremely inefficient at high-speed, low-torque conditions; electromagnetic motors are extremely inefficient at high-torque, low-speed conditions. The result of inefficiencies in the drive systems of conventional biped designs is that such robots are typically unable to traverse large distances or walk for long periods of time with the amount of energy that they are able to carry, e.g. in a battery. This limits the utility of bipeds for realistic applications in which they will be required to operate for long periods of time and at large standoff distances without the opportunity to recharge or replace batteries.
A second limitation of conventional biped design is that most conventional drive systems have high intrinsic mechanical impedance (mechanical impedance is the frequency dependent ratio of force or torque in response to a velocity applied at the output), meaning that they are intrinsically ineffective at applying controlled torque or force, which is required for the most efficient dynamic walking strategies. To achieve adequate torque control requires elaborate sensors and computation, and this approach is fundamentally vulnerable to contact instabilities and other performance limitations.
A need remains, therefore, for a humanoid biped robot that is able to power efficiently traverse long distances and operate for long periods of time under power from a source (e.g. a battery) that it can carry, reserving enough energy to perform manipulation or other activities required to complete relevant tasks. Secondarily, there is a need for a humanoid biped robot that achieves high-quality, highly-stable torque control and is able to stably implement the most aggressive, highest-performing and most-energy efficient emerging locomotion behavior algorithms.